       Re: Skipping Elements in Sum

• To: mathgroup at smc.vnet.net
• Subject: [mg131801] Re: Skipping Elements in Sum
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Mon, 7 Oct 2013 08:25:21 -0400 (EDT)
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• References: <diabl5\$ihc\$1@smc.vnet.net>

```To keep the same structure as Sum when there are no exclusions and to
handle exclusions with infinite sums

Clear[mySum]

mySum[expr_, {n_, min_, max_, step_: 1, excl : _List : {}}] :=
If[(max == Infinity) || (min == -Infinity),
Sum[expr, {n, min, max, step}] - Sum[expr, {n, excl}],
Sum[expr, {n, Complement[Range[min, max, step], excl]}]]

mySum[f[n], {n, 1, 10}]

f + f + f + f + f + f + f + f + f + f

mySum[f[n], {n, 2, 10, 2}]

f + f + f + f + f

mySum[x^n/n!, {n, 0, Infinity, 2}]

Cosh[x]

mySum[x^n/(-n)!, {n, -Infinity, 0, 2}]

Cosh[1/x]

If exclusions are given, then step must be included and exclusions must be
a list

mySum[f[n], {n, 1, 10, 1, {5}}]

f + f + f + f + f + f + f + f + f

mySum[f[n], {n, 1, 10, 2, {3, 5}}]

f + f + f

mySum[x^n/n!, {n, 0, Infinity, 2, {1, 3}}]

-x - x^3/6 + Cosh[x]

mySum[x^n/(-n)!, {n, -Infinity, 0, 2, {-1, -3}}]

-(1/(6*x^3)) - 1/x + Cosh[1/x]

Bob Hanlon

On Sun, Oct 6, 2013 at 3:46 AM, <scottcnoble at gmail.com> wrote:

> On Sunday, October 9, 2005 2:00:05 AM UTC-4, qcade... at gmail.com wrote:
> > Does anyone know how to skip elements using the mathematica sum? e.g.
> > take the sum of all i, where i not equal to j.
>
> mySum[expr_,min_,max_,excl_] := Block[
>                                        {newlist,i,n},
>
>  newlist=Complement[Table[i,{i,min,max}],excl];
>                                        Sum[ expr[newlist[[i]]],
> {i,1,Length[newlist]}]
>                                      ]
> where "expr" is a function representing the argument of the sum,  "min" is
> the minimum value of the index, "max" is the maximum value of the index,
> and "excl" is the list of indices to exclude from the sum.
>
> You can do this with Product[] as well...
>
>
>

```

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